Studies are being made of the physicochemical and enzymatic properties of the dynein isoenzymes in sea urchin sperm flagella. Two isoenzymes have been isolated so far--dynein 1 which accounts for about 85% of the axonemal ATPase activity and is located in the arms on the doublet tubules, and dynein 2 which accounts for about 15% of the ATPase activity and whose location is unknown. When extracted with 0.6 M NaCl, dynein 1 is obtained, sediments at 21S, and has a molecular weight of 1,250,000. This 21S form of dynein 1 has a latent ATPase activity, and is believed to correspond to the intact function cross bridge that induces sliding between adjacent doublet tubules. The ATPase activity of dynein can be selectively inhibited by vanadate, which thus provides a means to distinguish between the actions of dynein and myosin in different forms of cell motility.